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FGM fabrication
In this chapter the description of the procedure to manufacture the functionally graded (FG)
materials is presented. In this research, FGM specimens were manufactured from the
mixture of ceramics and polymers. Alumina (Al2!) is used for the ceramic constituent."poxy#resin is used for the polymer constituent. $ased on the extensi%e literature re%iew in
&hapter 2, it was found that, among three effecti%e techni'ues used for manufacturing
FGMs namely, thermal spraying, powder metallurgy and infiltration techni'ues, the
ceramic#polymer FGMs can only e manufactured appropriately y using the infiltration
techni'ue. *herefore, a multi#step se'uential infiltration techni'ue is used to faricate FG
eam specimens made from alumina and epoxy#resin in this research. In this chapter, the
information in relation to FGM farication will e pro%ided in detail as follows+
.- pecimen farication using multi#step se'uential infiltration techni'ue
.2 Microstructure analysis
.! Alternati%e techni'ue of specimen farication
5.1 Specimen Fabrication using multi-step sequential infiltration
techniqueAlumina#epoxy composite eams were manufactured using a multi#step se'uential
infiltration techni'ue. *he earliest de%elopment of the techni'ue was proposed y &ichoc/i
et al. (&ichoc/i et al., -001) to produce graded composite materials.
*he /ey principle of the infiltration techni'ue is to ma/e an alumina piece containing a
graded networ/ of porosity which will e infiltrated y epoxy in the final step, in order to
otain alumina#epoxy composite specimens. $y using this techni'ue, the composite
specimens can e represented in the form of interpenetrating#networ/ (I3) structuredcomposites which contain two phases of materials that are alumina and epoxy phases. In
se%eral pre%ious in%estigations (&lar/, -002, rielipp et al., -00, 4ange et al., -005), it
was affirmed that the infiltration techni'ue is an effecti%e methodology that can e used for
producing graded composite materials where their material compositions are continuous.
&omposite materials ha%ing the interpenetrating networ/ structures are different from
traditional fire#matrix and particle#matrix composite materials in terms of compositional
distriution and pattern which then lead to different eha%iour when they are su6ected to
mechanical loads. *herefore, this chapter will gi%e o%er%iews of the production process for
ma/ing graded composite eams.
*he uni'ue ad%antage of the infiltration techni'ue is that it is suitale for ma/ing graded
composites with a wide range of material constituents. As mentioned in &hapter 2 of theliterature re%iew, for example, 3eurand et al. (3eurand et al., 2552) successfully
employed the infiltration techni'ue to produce ceramic#metal graded composites which
were used for in%estigating residual stress analysis. *o otain graded composites which
ha%e a large disparity in elastic properties, ceramic#polymer could e used as the material
constituents. *hus, in the in%estigations of *ilroo/ et al. (*ilroo/ et al., 2557), the
alumina#epoxy graded composite system in which material compositions were %aried along
an axial direction, was produced using the infiltration techni'ue for the purpose of
in%estigating crac/ propagation. In this research, the material compositions etween
alumina and epoxy of the FG eam specimens will e graded across the thic/ness direction
of the specimens. *he outline of the faricating process is illustrated in Fig. .-.
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Fig. 5.1 chematic of faricating process of graded composite specimen
5.1.1 Foam preparation
According to the schematic of the production process, it egan with the foam preparation
process. *his is to construct a porous networ/ structure inside the specimens. *he networ/
structure was achie%ed y using open#celled polyurethane foam (8) as an imprint for such
a structure. *he polyurethane foam used in this preparation was commercial foam whose
a%erage cell si9e was approximately 5.1 mm. *he 8 foam was cut to the re'uired si9es(length -55 mm and readth -2 mm), then the foam pieces were sliced to three different
thic/nesses (h), 2 mm, : mm and 7 mm. *here were many ways to slice the foam pieces in
order to otain a series of %ery thin foams. For instance, in the foam preparation process of
(*ilroo/, 255), the 8 foam was immersed with water and fro9en to retain shape efore
it was cut to si9e with a andsaw. *his method of cutting the foam did not yield a perfect
result ecause the cut foam pieces were wet and re'uired thawing and drying later. 8sually,
the drying process of the wet foam pieces too/ around -2 hrs to dry in an o%en at : 5&.
*herefore, in this farication, the 8 foam was cut and sliced y using a hot#wire cutter
that can e considered as a etter way, without re'uiring any process of free9ing, thawing
and drying.
*he foam pieces with different thic/nesses were placed in a uniaxial hot press andcompressed to the same constant thic/ness of - mm, producing a series of foam pieces of
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different density. In order to retain the compressed shape, a heat treatment cycle had to e
applied during this compressing of foam. A schematic of the foam compression process is
shown in Fig. .2. As seen in Fig. .2, a pressure of !55 /a was applied to compress the
foam pieces. *he temperature was ramped up to -15 5& with the increasing rate of -
5&;min, and was held for - hour efore letting the temperature cool down naturally to room
temperature. *he foam was then remo%ed from the hot press. 8ne%enness at the edges dueto thermal stress was found which was remo%ed efore eing utilised in the suse'uent
stages.
*he uniaxial hot press that was used in this foam preparation process is shown in Fig. .!.
It is seen that the rass spacer containing the foam pieces ha%ing different thic/nesses was
positioned etween the top and ottom platforms. $oth platforms were set to heat up to -15
5& using a temperature control ox. *he pressure of the hot press was controlled at !55 /a
y an hydraulic system. *he spacer under the mentioned pressure and temperature was held
for - hour in order to ensure that the foam pieces inside had retained the new shapes. *he
spacer was then allowed to cool down naturally efore remo%al of the compressed foam
pieces.
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5.1.2 Slip casting and drying processes
*o manufacture graded composite specimens, a slip casting process was used to create
alumina phases in specimens. A useful and appropriate optimum slip formula for the
infiltration techni'ue was determined y 3eurand et al. (3eurand et al., -000).
*herefore, the alumina slip used to manufacture the specimens in this research was
produced using the same formula as proposed in (3eurand et al., -000, *ilroo/, 255).
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A %ery good mixture of all ingredients (the li'uid components) with the alumina powder
according to the formula can e achie%ed y the aid of a magnetic stirrer (I"& magnetic
stirrer) and an ultrasonic ath (83I3I&). All of the chemical ingredients and alumina
powder were weighed se'uentially corresponding to the formula and mixed together in a
ea/er. A teflon#coated magnetic ar was added to the ea/er, which was then placed on
the magnetic stirrer. *he magnetic stirrer was operated for -5 minutes efore the ea/er
was mo%ed to the ultrasonic ath for -5 minutes. Dowe%er, one cycle of mixing, was found
to e insufficient to otain a good mixture. *herefore, the mixing process with the magnetic
stirrer and the ultrasonic ath was repeated again. *he mixing process was carried out at
least ! times in order to produce the strongest mixture of all ingredients. It was found that
the mixing process led to a reduction of large agglomerates and produced good dispersion.
*he alumina slip was prepared one day efore casting for ageing purposes. $efore the
alumina slip was used for casting in the following day, it was re#mixed or re#dispersed y
the magnetic stirrer and the ultrasonic ath again.
In the slip casting process, the casting mould, which consists of pieces of erspex, was used
to create the graded composite specimens. *he erspex was made from plaster of aris. *he
pieces of the erspex were fitted together to the dimension of the compressed foam (length
-55 mm and readth -2 mm). *hey were placed on a laster of aris loc/ in a %acuum
chamer, as shown in Fig. .. *he enefit of the laster of aris loc/ is that it can asor
moisture from the alumina slip and let the specimen dry appropriately. *o minimise the
prolem of adhesion etween the surfaces of the cast specimen to the mould walls, the
erspex pieces were lined with *eflon tape. *he filter paper (A?A3*"&) was placed
etween the compressed foam and the plaster of aris loc/. *his is done in order to a%oid
the li/elihood of contamination of the alumina slip y particles or solule ions from the
plaster. *o clearly understand the slip casting set up, the cross#section of apparatus isillustrated in Fig. ..
After preparing the casting mould, the series of compressed foam pieces were stac/ed into
the mould. *he highest density foam piece was placed in the mould first, followed y the
lighter ones. ?ue to the density of the foam pieces eing much lighter than that of the
alumina slip, the foam pieces would normally float in the slip causing the prolem of
discontinuity etween ad6acent layers. *o sol%e this prolem, there were se%eral attempts to
glue the foam pieces together in order to ensure phase connecti%ity across the stepinterfaces.
8sing glue to stic/ the foam pieces together was one of many attempts to create
the connecti%ity howe%er, the glue tended to cause warping of the foam, producing
delamination. Another attempt was to use a heat treatment with slight pressure to construct
the foam piece connection. Dowe%er, these foam 6oining techni'ues presented ao%e werenot effecti%e in producing a suitale ond. *hus a etter way to 6oin the foam pieces was to
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place the foam pieces so they were restrained slightly y the mould walls or the erspex
pieces without using glue and then apply a little pressure when placing them into the
mould. &onse'uently, good connecti%ity etween the ad6acent layers can e otained.
lip casting was performed in the %acuum chamer (4A$"&) y placing the mould
assemly into the chamer. At the eginning stage of the slip casting process, the alumina
slip was filled into the foam pieces at a slow filling rate and using a small amount of theslip otherwise, the foam pieces would ha%e floated on the surface. *o enhance infiltration
of the slip casting process after the slip was poured o%er the foam pieces the pressure of the
chamer was lowered to around -55 mar that led to air ules lea%ing the foam.
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ceramic phase. *he temperature profile used for the whole process of foam pyrolysis is
illustrated in Fig. . with the details of temperature increment. For example, starting with
the room temperature, the following temperature was used+ 2 5& to 2! 5& at !55&;hour
held for - hour (foam urnout), 2! 5& to :55 5& at !55&;hour held for - hour (inderurnout),
:55 5& to 155 5& at :55&;hour held for - hour (ash urnout). *he specimen was
allowed to cool down naturally to room temperature efore it was heated up again for sintering the ceramic phase at -:55 5&, held for - hour. *he temperature profile of sintering
process is shown in Fig. .1. *he furnace, (&eramic engineering furnace manufacturer,
ydney, Australia) as shown in Fig. .0, was used for oth urning out foam and sintering
ceramic processes. It was found that sintering reduces the dimensions of the specimen in
e%ery direction, and lea%es an alumina specimen with layers of graduated porosities.
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5.1.4 Polymer infiltration
*he alumina specimen ha%ing layers of graduated porosities was ready for polymer
infiltration, using a %acuum chamer, in order to otain an alumina#epoxy composite
specimen. *he alumina specimen was placed into a silicone mould which was suitale for
this process ecause the specimen can e remo%ed easily when polymer is cured. "poxy
resin ("pofix, truers, ?enmar/) was chosen to fill the graduated porosities inside the
alumina specimen under %arying pressures and was then cured at room temperature. $y
%arying the pressure, air ules were forced to lea%e the specimen and epoxy resin could
infiltrate the graduated porosities more easily. *he cycles of %arying pressure within a
range of -55 to -555 mar were re'uired until ensuring the amount of air ules was
minimised. *he next step was to lea%e the epoxy resin cured at room temperature for
around 2: hours. After curing, excess epoxy was remo%ed y polishing with coarse
sandpaper. Finally, the surfaces of specimen were ground and polished with diamond paste.
5.2 Microstructure Analysis*he graded composite eam specimens made from alumina#epoxy which were produced
using the ao%e procedure were selected randomly to section across the thic/ness direction
to examine the 'uality of the graded area. *hen the sectioned specimen was mounted y
epoxy resin efore grinding and polishing with sandpapers and diamond paste, respecti%ely.
For polishing with the diamond paste, the specimen was first polished using an automatic
polisher (4eco #5). *he used diamond paste has a diamond particle si9e of ! m which
was reduced to - m at the final step.
Images of the polished surfaces, otained y optical microscopy with a 3i/on 255
microscope and digital camera, were used to characterise the pure alumina and epoxy
phases o%er the graded region. ne such cross#section image was shown in Fig. .-5. A
measurement of areas, using the areal techni'ue, in the cross#section yielded the
percentages of material compositions in each graded region. *he top and ottom surfaces
were referred to as the pure alumina and epoxy resin layers, respecti%ely.
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Fig. .-- illustrates the whole material composition at the graded region of the specimen.
*his includes information aout percentages of alumina and epoxy resin in each layer
across the graded region.
As shown in Fig. .--+ the cross#section image, a few pores were found due to imperfect
epoxy infiltration. It was also seen that the %ariation of material compositions throughout
the cross#section presented in the form of layer or step changes rather than a continuouschange o%er the section. $y using the areal techni'ue, one can otain the percentages of
alumina and epoxy resin in each layer in the graded region. It is clearly seen that the top
layer was for a pure alumina and followed with (5> alumina#!5> epoxy), (:5> alumina#
75> epoxy) and (25> alumina#15> epoxy) layers, whereas, the ottom layer was for a
pure epoxy resin layer. According to the material compositions or material %olume fraction
throughout the cross#section, these were plotted and illustrated in Fig. .-2. *he area underthe
step graph as shown in Fig. .-2 matches the percentage of alumina (V c) the proportion
of epoxy resin is indicated y the complementary area of the graph.
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*he process of slip casting was further in%estigated. A microstructure analysis displayed
the result of an imperfectly graded composite specimen when the alumina slip was filled
into the compressed foam pieces with a high filling rate and too much slip. *he
microstructure of the imperfect specimen that was made y a high rate of alumina slip
infiltration is shown in Fig. .-!. It can e seen that there are a numer of uffer layers of
alumina across the specimen (seen in white colour layers). *his is due to the compressed
foamHs density eing much lighter than that of the slip. *herefore, the foam pieces easily
float on the slip etween layers so that the uffer layers are constructed. *he uffer layers
cause the prolem of porosities occurring inside the specimen, especially at the middle 9one
of the specimen. It is ecause the uffer layers ha%e loc/ed or sealed a way for the epoxy
resin to come into the lower 9one. *his can e rectified y filling the alumina slip into thefoam pieces with a slow filling rate. *he foam pieces were allowed to asor all of the slip
in the pre%ious infiltration efore more slip was added to complete the slip casting process.
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5.3 Alternatie technique of specimen fabrication*he principles and ideas of powder metallurgy and multi#step se'uential infiltration
techni'ues as presented in &hapter 2 are comined to propose an alternati%e techni'ue to
produce layered graded specimens. *he main ad%antage of the comined techni'ue is that it
does not re'uire the processes of slip casting and drying therefore, time ta/en to
manufacture specimens is greatly reduced. *he comined techni'ue utilises the fact thatsmall particles of mothalls can e used to mix with the ceramic powder in each layer with
different compositions in order to produce graded porosities. *he mothalls are urnt out at
low temperature during the sintering process this lea%es the ceramic specimen ha%ing
graded porosities. After sintering, the ceramic specimen is solidified and ready for the
process of polymer infiltration, conse'uently, the graded composite specimen made from
ceramic and polymer can e otained.
*his proposed idea of a comined techni'ue can e pro%en y manufacturing specimens
from the new process. Mothalls are selected to mix with ceramic powder as they can e
urnt out completely at around 255 o&. *he production process of the specimens using the
comined techni'ue egan with grinding mothalls to small particle si9es whose diameter
are around 5.#-.5 mm. *he 00.00> purity alumina powder (*aimicron *M#?A=, *aimei&hemical &o 4td, Eapan) was mixed with the ground mothalls in different ottles which
were desired to ha%e different material compositions. In this farication, the fi%e layer
composite specimens were re'uired with the percentages of materials in each layer eing
(-55 %ol.> alumina#5 %ol.> mothall) (25 %ol.> alumaina#15 %ol.> mothall) (:5 %ol.>
alumina#75 %ol.> mothall) and (5 %ol.> alumina#!5 %ol.> mothall). Dowe%er, the last
layer contains pure epoxy which can e created in the final step of polymer infiltration.
Dence, four ottles with different percentages of alumina powder and ground mothalls
were otained. *he alumina powder and ground mothalls with &M& onder powder were
then lended for e%ery ottle y using a all milling machine (tar Machinery ty 4td) as
shown in Fig. .-:. *he all milling machine was set to run for :# hours at 255 rpm in
order to otain a good mixture of the material compositions.
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After the materials in each ottle were mixed, they were rought to fill and form in a steel
die. Fig. .- (a) illustrates the steel die. Fig. .- () displays the press machine ("nerpac
-! series), which was operated at !5 /3 to compress materials in the steel die. For layered
graded specimens, it was noted that the mixed powders were stac/ed se'uentially layer y
layer with a layer#wise compositional distriution. *he first layer (at the ottom of the die)
was gi%en for pure alumina layer and followed with (25 %ol.> alumaina#15 %ol.>mothall) (:5 %ol.> alumina#75 %ol.> mothall) and (5 %ol.> alumina#!5 %ol.>
mothall), respecti%ely.
*he green compacted specimen was remo%ed from the die slowly with a small application
of force through the compressing ar of the die in order to a%oid crac/s. *he greenspecimen was then sintered using the high temperature furnace which is shown in Fig. .0.
*he increasing temperature profile of this sintering was used on the same principle as
presented in Fig. .1. ?uring the sintering, the mothalls were urnt out. After sintering,
one can otain the alumina specimen ha%ing graded porosities. *o complete the farication
process, the graded porosities inside the alumina specimen were infiltrated y epoxy#resin("pofix,
truers, ?enmar/) with the aid of %arying pressure in the %acuum chamer as
shown in Fig. .-7.
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*he layered graded specimen made from alumina and epoxy using the comined techni'ue
was finalised in a similar fashion to the infiltration techni'ue y grinding and polishing the
surfaces. *he specimen was then sectioned to examine the 'uality of material gradients
across the graded direction using optical microscopy. It was found that the specimen
otained %ia the comined techni'ue had much more porosity than that produced y the
multi#step se'uential infiltration techni'ue. *o show the comined techni'ue leading to
porosities inside the specimen, Fig. .- presents diagrams of layered graded specimen
farication %ia this techni'ue. *he diagrams can e used to explain the mechanisms of
graded construction in the specimen. *he process of layer preparation is shown in Fig. .-
(a). *he mixtures of ground mothalls and alumina powder are used to create layers
according to the desired layer#wise compositional distriution. *he sintering process leads
to mothalls eing urnt out and lea%es the alumina specimen with layers of graded
porosities. *he suse'uent step is to infiltrate epoxy into the graded porosities of thealumina
specimen as shown in Fig. .- (). It can e seen that the epoxy profile - cannot
pass into the following layer, ut the epoxy profile 2 can penetrate through the porosity
connection. Dowe%er, %ery few of the porosity connections are created inside the specimen
using the comined techni'ue. *herefore, there is much porosity left without epoxy
infiltration in the specimen as shown in Fig. .- ().
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?ue to few porosity connections, the final specimen produced from alumina and epoxyusing the comined techni'ue may ha%e a lot of porosities as seen in Fig. .-1. *he
specimen in this figure contains ! phases+ epoxy, alumina and porosity phases. nly the topand
ottom layers can e classified as the perfect layers which are the layers of pure epoxy
and pure alumina respecti%ely.
According to the diagrams of layered graded specimen farication, it can e concluded thatthe comined techni'ue which is de%eloped from the comination of ideas of powder
metallurgy and infiltration techni'ues is not suitale for manufacturing the layered graded
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specimen. *his is ecause the comined techni'ue cannot produce the interpenetratingnetwor/
(I3) structures which are %ery important to minimise the amount of porosity.
Dence, to produce the layered graded specimen made of alumina and epoxy, the multi#step
se'uential infiltration techni'ue as presented in ection .- is strongly recommended to e
used for producing such specimen. In Fig. .--, it is clearly seen that the interpenetratingnetwor/
(I3) structures are created in the specimen produced y the multi#step se'uentialinfiltration techni'ue and the numer of porosities is minimal. *hus, the layered graded
specimens faricated y the multi#step se'uential infiltration techni'ue are used for further
in%estigation of %iration testing in this research.